Method of deoxidizing metals



Feb. 14, 1967 F. s. DEATH ET L METHOD OF DEOXIDIZING METALS 3Sheets-Sheet 1 Filed Oct. 1, 1963 Arc Torch I Z? INVENTORS M FRANK s.DEATH 13 BRUCE C.WH|TMORE ATTORNEY Feb. 14, 1967 F. s. DEATH ETAL METHODOF DEOXIDIZING METALS 3 heets-Sheet :5

Filed Oct. 1,

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wm QM NM Om 0N QN .WN NN ON 1 NHQAXO INVENTORS FRANK S. DEATH BRUCEC.WH|TMORE ATTORNEY United States Patent O T 3,304,169 METHOD OFDEOXIDIZING METALS Frank S. Death, Tonawanda, N.Y., and Bruce C.Whitmore, Morris Plains, N.J., assignors to Union Carbide Corporation, acorporation of New York Filed Oct. 1, 1963, Ser. No. 312,999 Claims.(Cl. 75-10) This invention relates to a method for deoxidizing metalsand more particularly to such a process for de oxidizing metals having alow solubility for carbon. Typifying the metals contemplated by thisinvention are metals such as copper, silver and gold.

A particularly useful application of the process of the invention is incopper smelting. Accordingly, the discussion and following descriptionwill be directed to a process for deoxidizing copper which has utilityas a step in copper smelting. However, the scope of the invention is notto be interpreted as being restricted to this specific metal.

Conventional copper smelting involves several pyrometallurgical steps inwhich copper concentrate melting first takes place to separate theiron-copper sulfides from gangue minerals. Next, exothermic air blowingin separate vessels called converters is employed to oxidize the sulfurand iron, with sulfur being removed in the form of sulfur dioxide gasand iron going into the slag as ferrous oxide. The remaining copper,called blister copper, contains small amounts of sulfur and iron; alongwith silver, gold and trace amounts of impurities such as arsenic,antimony, bismuth, lead, selenium and tellurium. Ordinarily, refining ofblister copper involves the following sequence of operations: (1) firerefining in a reverberatory furnace from which anodes are cast forfurther purification by electrolytic techniques, (2) electrolyticrefining to recover precious metals and remove impurities, and (3) asecond fire refining to produce copper with the physical propertiesrequired for industrial use.

In fire refining, the blister copper is oxidized by air blowing toremove traces of sulfur, iron, and other impurities. The dross-like slagwhich is formed is rabbled off (removed with an iron scraper or rakecalled a rabble) to be recycled to the converter. At this point, themelt, termed set copper, will have an oxygen content of up to 0.8%,which is contained as atomic oxygen at the fire-refining temperatures.On solidification, a cop- .per-cuprous oxide eutectic is formed whichhampers further refining. Accordingly, the oxygen content hastraditionally been lowered to the range of 0.20.3% by poling, that is,by thrusting green tree trunks into the batha technique which is dirty,dangerous and archaic, although up until now, difficult to improve on.'In poling, the heat of the bath causes destructive distillation of thewood with the evolution of hydrogen, hydrocarbons, carbon monoxide andwater vapor. These gases cause violent agitation of the bath and bringabout effective deoxidation.

Following the initial fire refining and poling, anodes are cast, andthese are subjected to electrolytic refining in an electrolysis processin which copper goes into solution from the anode and is redeposited ina purer form at the cathode. The cathodes from the electrolytic refiningare usually remelted and subjected to a second fire refining in whichadditional deoxidation may be required. If remelting is done in aconventional furnace, then oxidation of the melt results from contactwith the atmosphere so that poling is again required. The product fromthis poling step is termed electrolytic tough-pitch copper and will havean oxygen content of 0.02 to 0.05%.

Accordingly, it is a main object of the invention to provide a processfor reducing the oxygen content of an oxidized metal having a lowsolubility for carbon.

3,304,169 Patented Feb. 14, 1967 Another object is to provide a processfor the deoxidization of copper.

Yet another object is to provide an electric arc process for deoxidizingcopper wherein a hydrocarbon gas is introduced into the arc effluentproviding the deoxidizing agent.

Still another object is to provide a simple convenient method foreliminating poling in conventional copper melting.

These and other objects will become apparent from the followingdescription and drawings in which:

FIGURE 1 is a cross-sectional view of typical apparatus for carrying outthe invention;

FIGURE 2 is a partial cross-section of a typical arc device for carryingout the invention;

FIGURE 3 is a graph showing deoxidization curves and carbon dioxide tocarbon monoxide ratio obtained using the process of the invention; and

FIGURE 4 is a deoxidization curve obtained using the process of theinvention on a larger quantity of copper.

Generally, the objects of the invention are accomplished by providingand maintaining a molten metallic bath containing the metal to bedeoxidized, establishing an electric are between a non-consumableelectrode and said molten metallic bath, flowing a hydrocarboncontaining gas through a nozzle having at least a central bore thelongitudinal axis of which is aligned with the arc column; introducingthe hydrocarbon containing gas into the arc column; directing thehydrocarbon gas containing arc column directly onto the surface of themolten metallic bath whereby the highly reactive products from thehydrocar bon react at the metal surface with the oxygen contained in themolten metallic bath.

In the broadest aspects, the invention contemplates introducing thehydrocarbon gas into the arc column before the arc passes through thenozzle as well as introducing the hydrocarbon .gas into the arc columnafter it passes through the nozzle.

The invention is predicated on the discovery that when a hydrocarbontgas, typically methane is intimately mixed with the plasma of adirectionally stable arc the dissociation product from the hydrocarbon,gaseous carbon and hydrogen are transported in a highly reactive stateby the arc plasma to the melt surface and react with oxygen whichdiffuse from the bath to form reaction products.

For purposes of this disclosure, the term directionally stable arc isdefined as an arc column the longitudinal axis of which will maintainthe direction originally imparted to it.

There are several methods of producing and maintaining directionallystable arcs. Examples of directionally stable arcs of the typecontemplated by our invention are described in Gage, US. Patent2,806,124 and in US. patent application Serial No. 223,484, filedSeptember 13, 1962, to Robert J. Baird.

The advantages of using a directionally stable arc include providing asimple method of controlling power to the bath by changing arc length;providing an arc column which can be directed to the location desiredand providing a means for transporting the deoxidizing agents to themolten bath in a highly reactive state.

Typical apparatus for carrying out the invention is shown in FIGURE 1. Atypical example of the invention will be described in referring toFIGURE 1 so that those skilled in the art might clearly understand howto carry out the process of the invention.

Referring now to FIGURE 1, a device for producing a directionally stableelectric arc is positioned in the roof R of a furnace F. A bottomelectrode is positioned in the bottom of the furnace. Power for theelectric arc is provided by source P, such as a conventional directcurrent power supply. The particular furnace design is any suitableelectrical furnace and forms no part of the present invention.

In greater detail, a typical device for producing a directionally stablearc is shown in FIGURE 2. In this figure, an electrode 1 is carried by ahollow holder 3 having a baffle 5 positioned therein. Cooling waterenters the baffie and leaves the holder through the passage 7 formed bythe batiie and walls of the hollow holder. A first tubular member 9surrounds the electrode holder and forms therewith an arc gas passage11. A second'tubular member 13 surrounds the first tubular member andforms a hydrocarbon gas passage 15 therebetween. This entireconfigoration is surrounded by a Water jacket 17 having passages 19 and21 for water in and water out, respectively. Depending from theplurality of tubular members and sealing the forward end of the Waterjacket 17 is a nozzle 23 having passages 25 arranged symmetrically andwhich are in registry with the hydrocarbon passages 15. Also provided inthe torch is a central bore 27, the longitudinal axis of which is inaxial alignment with the electrode 1.

In actual operation, the copper to be deoxidized is provided to thehearth H of the furnace F. A transferred straight polarity DC. are isestablished between electrode 1 and the bottom electrode through themetal charge. The necessary power required to bring the copper totemperature, about 1200 C., is provided by the directionally stable arcand control over the power is obtainable simply by varying the arclength or the current of the directionally sta'ble arc. An arc gas suchas for example argon, helium, nitrogen, hydrogen and carbon monoxide ispassed through passages 11 and together with the arc passes out of thetorch through the bore 27 of nozzle 23 as an arc plasma.

It is possible and in fact is preferred to use the hydrocarbon gas asthe arc gas and eli-m-inate the need for argon. In such case, thehydrocarbon gas would be introduced into the arc column before suchcolumn passes through the nozzle 23. The important criteria in eithercase is that the hydrocarbon be introduced into the arc and that thedissociation hydrocarbon products be carried in a highly reactive stateto the molten metal surface by such arc.

The are plasma carrying the reactive products of hydrocarbondissociation are directed to and directly impinge on the molten copperbath where deoxidization takes place and the oxygen readily diffusesfrom the bath to the surface. The presence of a slag layer on the bathmust be avoided since direct contact of the arc plasma with the moltencopper is essential. Most of the deoxidation occurring in this processis attributable to carbon, although both carbon and hydrogen function asreducing agents. Carbon is only slightly soluble in molten copper sothat it does not permeate the melt but reacts with the oxygen at themelt surface.

The novel method of deoxidizing molten copper disclosed herein providesa preferred alternative to the conventional technique of paling which iscapable of producing not only tough-pitch copper but alsooxygenfree-high-con-ductivity copper. Arc torch deoxidation of coppernot only eliminates such undesirable features of poling as the possiblehazard from the violent action within the furnace and the necessity forstoring and handling wooden poles and provides instead a flexible,efiicient and easily controlled process; but also provides a verysignificant reduction in operating costs.

The following is an example of experiments wherein oxygen content ofcopper was reduced to very low levels, less than 0.02% oxygen. Fortypounds of copper having an oxygen content of over 0.5% were melted witha transferred straight polarity are operated with an argon flow of 90 to95 c.f.h. at 40 volts. Methane was then introduced through gas passagesarranged symmetrically around the bore of the nozzle at a fixed flowrate to give a total gas flow of 100 c.f.h. The voltage was 50 volts.Deoxidation progress was followed both by gas chromatographic analysisof the off-gas and by taking pin-tube samples from the bath. FIGURE '3shows the oxygen content as a function of time for each of twodeoxidati-on tests making up this experiment. For the run in which 5c.f.h. methane and 95 c.f.h. argon was used, it can be seen that theoxygen was reduced to less than .-1% oxygen. Chromatographic analyseswere ob-- tained and the ratio of CO /CO in the offlgas is also shown.Results obtained with 10 c.f.h. methane and c.1..h. argon are alsoplotted.

Consideration of the results shown in FIGURE 3 indicates that theprocess of this invention provides an effective means for rapidlyreducing the oxygen content of molten copper to very low levels.

A second experiment analagous to the one just described except that afifty pound rather than a forty pound melt was used. FIGURE 4 shows theoxygen content of the bath as a function of time for the deoxidationstep utilizing 5 c.f.h. of CH; and 9S c.f.h. argon.

Operating variables in an arc deoxidation process include total are gasflow rate and the ratio of hydrocarbon or reactive gas to inert gas. Theexperimental results that have been obtained indicate that even with amoderate plasma flow, it should be possible to deoxidize to the requiredoxygen level in a period that is short relative to the time required forpoling. Thus, it would not be necessary to operate the torch at veryhigh reactive gas flow rates and the total flow could be set at thatlevel which gives optimum arc characteristics. Increasing the ratio ofhydrocarbon gas to argon in the plasma at a fixed total flow rate wouldalso decrease the time for deoxidizing a bath. More importantly,increasing the proportion of hydrocarbon results in improvement in theeconomics of the process, since it minimizes the requirement for argon.The optimum process would be that in which the torch is operated with agas consisting entirely of hydrocarbons, thus eliminating the argon costentirely.

What is claimedis:

1. Method of deoxidizing metals taken from the group consisting ofcopper, silver and gold which comprises providing and maintaining amolten metallic bath containing said metal to be deoxidized,establishing an electric arc between a nonconsuma-ble electrode and saidmolten metallic bath, supplying an arc gas into said arc, flowing ahydrocarbon contanin-g gas through a nozzle having at least a centralbore, the longitudinal axis of which is aligned with the arc column, andintroducing said hydrocarbon containing gas into said are column,directing said hydrocarbon gas containing arc column directly onto thesurface of said molten metallic bath, whereby the carbon deoxidizes themetal being treated.

2. A method of deoxidizing metals taken from the group consisting ofcopper, silver and gold which comprises providing and maintaining amolten metallic bath containing. said metal to be deoxidized,establishing an electric are: between a non-consumable electrode andsaid molten. metallic bath, flowing an arc gas along said non-consum'able electrode, surrounding at least a portion of said arc and flowingarc gas with a water-cooled nozzle, introducing a hydrocarbon gas intothe arc and are gas stream, impinging the hydrocarbon gas containing arceflluent onto the surface of the melt to be deoxidized whereby at leastthe carbon acts to deoxidize said metal.

3. Method of de-oxidizing metals taken from the group consisting ofcopper, silver and gold which comprises providing and maintaining amolten metallic bath containing said metal to be deoxidized,establishing an electric are between a non-consumable electrode and saidmolten metallic bath, introducing a hydrocarbon'gas into said arc,passing said hydrocarbon gas containing arc column through a nozzlehaving at least a central bore the longitudinal axis of which is alignedwith the are column, directing said hydrocarbon gas containing arccolumn directly onto the surface of said molten metallic 5 bath wherebythe carbon deoxidizes the metal being treated.

4. Method according to claim 3 wherein the hydrocarbon gas is methane.

5. Method of deoxidizing metals taken from the group consisting ofcopper, silver and gold which comprises providing and maintaining amolten metallic =bath containing said metal to be deoxidized,establishing an electric are between a non-consumable electrode and saidmolten metallic bath, supplying an arc gas taken from the classconsisting of argon, helium, nitrogen, hydrogen, and carbon monoxideinto said are column, flowing a hydrocarbon gas through a nozzle havingat least a central bore,

6 the longitudinal axis of which is aligned with the arc column,introducing said hydrocarbon gas into said are column, directing saidhydrocarbon gas containing arc column directly onto the surface of saidmolten metallic bath whereby the carbon deoxidizes the metal beingtreated.

References Cited by the Examiner UNITED STATES PATENTS 2,989,397 6/1961Kuzell et a1. 7576 DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

1. METHOD OF DEOXIDIZING METALS TAKEN FROM THE GROUP CONSISTING OFCOPPER, SILVER AND GOLD WHICH COMPRISES PROVIDING AND MAINTAINING AMOLTEN METALLIC BATH CONTAINING SAID METAL TO BE DEOXIDIZED,ESTABLISHING AN ELECTRIC ARE BETWEEN A NON-CONSUMABLE ELECTRODE AND SAIDMOLTEN METALLIC BATH, SUPPLYING AN ARE GAS INTO SAID ARE, FLOWING AHYDROCARBON CONTAINING GAS THROUGH A NOZZLE